This invention relates to an apparatus and a method of monitoring waveforms representing the respiration of a patient, and of detecting waveforms having characteristics indicative of apnea, this method comprising processing a first electrical signal having a waveform representing the respiration of the patient and a second electrical signal representing the electrocardiogram of the patient.
In the method of monitoring waveforms representing a patient's respiration, the reliability of apnea alarms (indicating cessation of respiration), accuracy of measurement of the duration of each apnea, and the sensitivity of the measuring system for detecting apnea, are greatly restricted by the presence of artefacts, which are synchronous with cardiac activity, on the wave of the signal representing the patient's respiration, this signal being produced, for example, by measuring the variation in the transthoracic impedance. In this case, the artefacts, known as cardiovascular artefacts, are due to the fact that the movement of the heart and the variation in blood flow produce a variation in the impedance of the rib cage.
If the respiratory signal wave did not contain the above artefacts, apnea could be detected, at least in principle, by monitoring the amplitude of the respiratory signal wave and by detecting the intervals in which said amplitude is below a predetermined threshold value. In actual fact, a method of this kind is not reliable, because the amplitude of the cardiovascular artefacts, which are present even in the case of apnea (an apnea should be indicated by a shallow respiration wave of practically zero amplitude), may very often exceed the selected detection threshold and may therefore erroneously be thought to indicate the presence of normal respiration. If a low detection threshold is selected, in fact, there is a risk of apnea occurring unnoticed. If, on the other hand, a high detection threshold is chosen, there is a risk of false apnea alarms being triggered when the respiratory signal is of low amplitude but not yet low enough to be an apnea. Since the cardiovascular artefacts may often have an amplitude equal to or even greater than that of the respiratory signal if the latter is shallow, the user of such a method finds it impossible to choose a suitable detection threshold to enable him to obtain reliable apnea indication and to measure the duration of such apnea accurately.
Fluctuations in the baseline of the respiratory signal and other interference signals frequently superimposed thereon also make reliable detection of apnea difficult.
The following methods have already been proposed to improve the reliability of apnea detection despite the presence of cardiovascular artefacts:
In a first known method (French patent application published under the No. 2,267,734), the interval of time corresponding to the period of the respiratory signal is compared with the interval between successive QRS complexes of the signal representing the patient's electrocardiogram. and if the difference between these intervals remains below a predetermined threshold value over a predetermined interval of time it is assumed that the apparently existing respiratory signal is in fact a cardiovascular artefact.
In a second known method (French patent application published under the number 2 192 790) the phase difference is measured between the respiratory signal and the signal representing the patient's electrocardiogram, and if this difference remains below a predetermined threshold value over a predetermined interval of time it is assumed that the apparently existing respiratory signal is in fact a cardiovascular artefact.
The above two known methods have the following disadvantages:
Since cardiac artefacts are often at the limit of detection, their random detection eludes the frequency or phase comparison means and results in respiration being thought to be normal, with no apnea detection.
In a third known method (UK patent application No. 2 060 892), a signal is formed which represents the first derivative of the respiratory signal with respect to time and the slope of this signal is examined at intervals of time corresponding to the intervals of time between successive QRS complexes of the signal representing the patient's electrocardiogram, and if this slope assumes a negative value in the interval during which it is examined, and if this occurs a plurality of consecutive times, it is assumed that the apparently existing respiratory signal is in fact a cardiovascular artefact.
This third known method has the following disadvantages:
it is sensitive to the polarity of the signal and does not operate if the electrodes are not correctly placed,
it is sensitive to noise of any kind and, in particular, a very noisy random signal may be classed as apnea because it has every likelihood of having negative slopes in the examination interval,
it takes into account the form of the artefact at the time of incidence of the QRS complex, such form is however not identical from one patient to another and depends on the positioning of the electrodes.
This method may therefore be inoperatve on certain patients.